Tissues: Structure, Function, and Classification in Human Anatomy & Physiology

Topic #2, Part 1: Tissues

Introduction to Tissues

Tissues are groups of structurally similar cells that perform a common or related function. The organization of cells into tissues is essential for the division of labor and cooperation at the organ level, contributing to overall homeostasis in the human body.

• Definition: A tissue is a group of cells with similar structure and function.

• Importance: Specialized cells allow for efficient division of labor and organ function.

• Histology: The study of tissues and their cellular organization.

The Four Primary Types of Tissues

The human body is composed of four main tissue types, each with distinct roles:

• Epithelial tissue: Covers body surfaces and lines cavities; forms boundaries.

• Connective tissue: Supports, binds, and protects other tissues and organs.

• Muscle tissue: Responsible for movement.

• Nervous tissue: Controls and regulates body functions.

Epithelial Tissue

General Characteristics and Functions

Epithelial tissue consists of sheets of cells that cover surfaces or line cavities, creating boundaries between different environments. It is highly specialized for various functions.

• Protection: Shields underlying tissues from mechanical, chemical, and infectious damage (e.g., skin).

• Absorption: Uptake of substances (e.g., GI tract).

• Filtration: Selective movement of substances (e.g., kidney).

• Excretion: Removal of waste products (e.g., kidney).

• Secretion: Release of substances (e.g., glands, GI tract).

• Sensory reception: Detection of stimuli (e.g., taste buds, olfactory membranes).

Special Structural Characteristics of Epithelial Tissue

• Polarity: Epithelial cells have an apical (top) and basal (bottom) surface, each with distinct structures and functions.

• Specialized Contacts: Cells are tightly joined by junctions (tight junctions, desmosomes) to maintain integrity and control permeability.

• Supported by Connective Tissue: The basement membrane (composed of basal lamina and reticular lamina) anchors epithelium and provides support.

• Avascular but Innervated: Epithelial tissues lack blood vessels but are supplied by nerves; nourishment comes from underlying connective tissue.

• Regeneration: High capacity for renewal due to frequent cell loss and damage.

Additional info: Cancerous epithelial cells often lose the ability to respect boundaries imposed by the basement membrane, contributing to metastasis.

Classification of Epithelial Tissue

Epithelial tissues are classified based on two criteria: cell shape and number of layers.

• Cell Shape: Squamous (flat), cuboidal (cube-shaped), columnar (tall).

• Number of Layers: Simple (one layer), stratified (multiple layers).

Types of Simple Epithelia

• Simple Squamous Epithelium: Thin and permeable; functions in filtration and diffusion. Locations: Endothelium (lining of blood vessels), kidney, lungs.

• Simple Cuboidal Epithelium: Secretion and absorption. Locations: Kidney tubules, small glands.

• Simple Columnar Epithelium: Secretion and absorption. Locations: Most of the digestive tract.

• Pseudostratified (Ciliated) Columnar Epithelium: Appears stratified but is a single layer; often contains cilia and mucus-secreting cells. Locations: Respiratory tract.

Types of Stratified Epithelia

• Stratified Squamous Epithelium: Protective role; basal cells are cuboidal and undergo mitosis, apical layers are flattened. Locations: Skin, lining of mouth.

• Transitional Epithelium: Stratified epithelium with a mix of columnar and squamous cells; adapts to stretching. Locations: Urinary bladder.

Glandular Epithelia

Glands are epithelial structures that produce and secrete products.

• Endocrine Glands: Ductless; secrete hormones directly into the bloodstream.

• Exocrine Glands: Secrete products into ducts; include sweat, oil, salivary glands, liver, pancreas.

• Unicellular Exocrine Glands: Single cells, such as goblet cells, secrete mucus.

• Multicellular Exocrine Glands: Composed of a duct and secretory unit, supported by connective tissue.

Modes of Secretion

• Merocrine: Secretion by exocytosis (e.g., salivary glands, most sweat glands).

• Holocrine: Secretion by cell rupture (e.g., sebaceous glands).

• Apocrine: Secretion by pinching off the cell apex (e.g., mammary glands).

Connective Tissue (CT)

General Functions and Types

Connective tissue is the most abundant and widely distributed tissue type, providing support, protection, insulation, and transportation.

• Binding & Supporting: e.g., tendons, ligaments.

• Protecting: e.g., bone, cartilage.

• Insulating: e.g., adipose tissue.

• Transporting: e.g., blood.

Major types: Mesenchyme, CT proper, cartilage, bone, blood.

Clinical Note: Rheumatoid arthritis is an autoimmune disease where the immune system attacks synovial membranes of joints.

Structural Organization of Connective Tissue

• Ground Substance (Matrix): Composed of interstitial fluid, cell adhesion proteins (e.g., fibronectin, laminin), and proteoglycans (act as molecular sieves).

• Fibers:

  • Collagen fibers: Provide high tensile strength.

  • Elastic fibers: Contain elastin; allow stretch and recoil.

  • Reticular fibers: Thin, form fine networks for support.

• Cells:

  • "Blast" cells: Immature, actively dividing and synthesizing matrix (e.g., fibroblast, chondroblast, osteoblast).

  • "Cyte" cells: Mature, maintain tissue (e.g., fibrocyte, chondrocyte, osteocyte).

Types of Connective Tissue Proper

Loose Connective Tissue

• Areolar CT: Gel-like matrix with all fiber types; widely distributed under epithelia. Functions: Cushions organs, immunity, inflammation, fluid reservoir.

• Adipose CT: Modified areolar CT; stores nutrients in adipocytes. Functions: Fuel reservoir, insulation, supports and protects organs. Locations: Under skin, around kidneys and eyeballs, in bones.

• Reticular CT: Contains only reticular fibers. Functions: Forms soft internal skeleton supporting free blood cells. Locations: Lymph nodes, bone marrow, spleen.

Dense Connective Tissue

• Dense Regular CT: Bundles of collagen fibers running parallel; great resistance to tension. Locations: Tendons, ligaments, aponeuroses.

• Dense Irregular CT: Collagen bundles arranged irregularly; withstands tension in many directions. Locations: Dermis, submucosa of GI tract, fibrous capsules of organs.

• Elastic CT: High content of elastic fibers; found in elastic ligaments.

Other Connective Tissues

• Cartilage, bone, and blood will be covered in subsequent sections.

Key Questions and Review

• Which fibrous element gives CT high tensile strength? Collagen fibers.

• Which type of CT can increase in volume without making more cells? Areolar CT.

• Which kind of CT acts as a sponge, soaking up fluid during edema? Areolar CT.

Table: Epithelial Types and Locations

Location	Epithelial Type
Lines stomach, most of intestines	Simple columnar
Lines inside of mouth	Stratified squamous
Lines much of respiratory tract	Pseudostratified ciliated columnar
Endothelium	Simple squamous
Composes lung alveoli	Simple squamous

Table: Connective Tissue Cell Types

Tissue Type	"Blast" Cell	"Cyte" Cell
Connective Tissue Proper	Fibroblast	Fibrocyte
Cartilage	Chondroblast	Chondrocyte
Bone	Osteoblast	Osteocyte
Blood	Hematopoietic stem cell	Blood cells (RBCs, WBCs)

Key Formula

General formula for tissue classification:

$\text{Tissue Type} = \text{Cell Shape} \times \text{Number of Layers}$

Additional info: This formula is a conceptual representation for epithelial tissue classification.